Over recent years, the interest in semiconductor nanowires has intensified as nanotechnology becomes an important engineering discipline. Nanowires, which are also referred to as nanowhiskers, nanorods, nanopillars, nanocolumns, etc. by some authors, have found important applications in a variety of electrical and optoelectrical devices such as sensors, solar cells to LEDs.
For the purpose of this application, the term nanowire is to be interpreted as a structure being essentially in one-dimensional form, i.e. is of nanometer dimensions in its width or diameter and its length typically in the range of a few 100 nm to a few μm. Usually, nanowires are considered to have at least two dimensions not greater than 500 nm, such as not greater than 350 nm, especially not greater than 300 nm such as not greater than 200 nm.
Many different types of nanowires exist, including metallic (e.g., Ni, Pt, Au), semiconducting (e.g., Si, InP, GaN, GaAs, ZnO), and insulating (e.g., SiO2, TiO2) nanowires. The present inventors are primarily concerned with semiconductor nanowires although it is envisaged that the principles outlined in detail below are applicable to all manner of nanowire technology.
Conventionally, semiconductor nanowires have been grown on a substrate identical to the nanowire itself (homoepitaxial growth). Thus GaAs nanowires are grown on GaAs substrates and so on. This, of course, ensures that there is a lattice match between the crystal structure of the substrate and the crystal structure of the growing nanowire. Both substrate and nanowire can have identical crystal structures. The present invention, however, concerns nanowires grown on graphitic substrates.
Graphitic substrates are substrates composed of single or multiple layers of graphene or its derivatives. In its finest form, graphene is a one atomic layer thick sheet of carbon atoms bound together with double electron bonds (called a sp2 bond) arranged in a honeycomb lattice pattern. Graphitic substrates are thin, light, and flexible, yet very strong.
Compared to other existing transparent conductors such as ITO, ZnO/Ag/ZnO, TiO2/Ag/TiO2, graphene has been proven to have superior optoelectrical properties as shown in a recent review article in Nature Photonics 4 (2010) 611.
The growth of nanowires on graphene is not new. In WO2012/080252, there is a discussion of the growth of semiconducting nanowires on graphene substrates using molecular beam epitaxy. WO2013/104723 concerns improvements on the '252 disclosure in which a graphene top contact is employed on nanowires grown on graphene.
For many applications it will be important that the nanowires can be grown perpendicular to the substrate surface. Semiconductor nanowires normally grow in the [111] direction (if cubic crystal structure) or the [0001] direction (if hexagonal crystal structure). This means that the substrate surface needs to be (111) or (0001) oriented where the surface atoms of the substrate is arranged in a hexagonal symmetry.
One problem, however, is that it is difficult to nucleate a nanowire on a graphene substrate. As the surface of graphene is free of dangling bonds, it is difficult for any nanowire to grow. The graphene is also inert making any reaction between the growing nanowire and the substrate unlikely. The present invention relates, inter alia, to functionalization of the graphene surface or to the inclusion of new layers or small islands on top of the graphene surface to enhance nucleation of nanowires thereon. The inventors still benefit, however, from the remarkable properties of graphene in terms of its strength, flexibility, transparency and electrical conductivity.
The present inventors have surprisingly found that improvements in nanowire or nanopyramid nucleation can be achieved in various ways.